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Ok a little later I will give my impressions of mu,pent mu, normal and interstage stages in a phono amp. The short answer is the interstages make a beautiful full dynamic sound.
Now for the question. I put the interstages in and have been trying different tubes in 1st and 2nd position with different b+'s and currents. I was thinking about it more that the 1m resistor was still in between the the 1st and 2nd interstage and the 2m resistor was in between the 2nd and 3nd interstage.
In the rest of the non phono designs I have not used load resistors because everyone said they are not needed. They said the impedance of the windings act like a plate choke/grid choke and create a very large load.
I am trying to understand how the previous stage see the load thru the grid? If someone could really break this down into elementary terms it would be great.
Also short of a manor change in the riaa, any problems with the removal of these resistors in a split riaa phono amp?
Thanks Tom
Follow Ups:
I am trying to understand how the previous stage see the load thru the grid? If someone could really break this down into elementary terms it would be great.
I'm not sure what you are asking but on the Innerstage type phono stage like in this schematic, G2 is tied to the plate and loaded thru the primary of the LL1660 in each case which is essentially AC ground once its hooked to its DC supply source.On G1 of the first tube,it loads thru a 47k to ground. G1 on the second tube goes thru a 220k and then grounds thru the secondary of the first innerstage. I didn't see your schematic so I can only assume a small picture of what you are saying.
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Mike,
Did you get this scheme form Mr. Ciuffoli's web site? I have not been there in a few years. His interesting work inspired to try Lundahl Iron.
Cool stuff...of coarse AM core interstage will put ya back money wise...especially for four of those puppies...
Stuben
" AM core interstage will put ya back money wise...especially for four of those puppies..." Don't I know that! My phono pre has four, line stage two, and mono-block power amps six if you count the CT plate chokes and output trannies. Then there are six AM core input transformers also in the mix. Needless to say, Kevin Carter is now a good friend of mine. ;~)
Nah
I found it on line.
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Hi Michael
I did not use the correct terms most likely. The above schematic uses single stage riaa where I use a split riaa like a Morgan Jones etc... I think you hit on a key here with the AC flowing in a certain direction. The large 220k resistor in the above schematic is in series and I think it is a part of the riaa network. The resistors I am talking about are shunt resistors which you see in most amp designs when you are not using interstage transformers. They are normally very large 1m and up. It was my understanding they where used to load the stage(This is where I might be wrong) The example above would be the standard 47k cart load to ground.
The 47k load is there to impedance match a standard mm cart. In the above schematic G1 on the second tube you state goes thru the 220k series resistor and also shunts to ground thru the secondary winding on the 1st interstage??? So does G1 on second tube see load combination of 220k and the secondary winding? (which is acting like a grid choke) This is where I
am confused. For example my 1st tube is plate loaded with the 1660 interstage then it goes thru a 100k series resistor a 2k series resistor and then a 600pf cap to ground. Which is 75 correction of the riaa. The grid circuit is connected between 100k and 2k resistor to the grid of the 2nd tube with that 1m shunt resistor to ground.
So I was under the assumption that this 1m resistor was there to load the grid of the second tube. I also read that if you use interstage transformers the secondary winding acts as a grid choke and that is why you normally do not see these big shunt resistors when using them.
I am all ears here I am a little confused on the ac path.
Thanks Tom
Are you talking about "grid leak" resistors? They help set the input impedance of the tube they are connected to. While high input impedance is a good thing, !M can be a problem. When tubes age they can get gassy and experience grid emissions (current). This flows through the grid leak resistor to ground and drives the grid positive (Ohms law). That drives the tube harder (more plate current). Not a stable condition. One of the benefits of ITs is that they provide a low DCR to ground for the tube's grid. Grid chokes can do the same thing. It would be really helpful if you could post a schematic of what you are trying to do. In phono pre's RIAA considerations come into play. My phono pre is 3 stage differential with the 1st/2nd stage direct coupled, 2nd/3rd IT coupled with a line out transformer. All RIAA resistors and caps are shunt. Getting the RIAA response correct is a bit tricky as Miller capacitance etc. come into play. I'm sure if you can post a schematic you'll be able to get some help and advise.
I do not have a schematic program so a schematic is a little difficult. The phono is already going. What I did was use my normal phono stage which was resistor plate loaded and cap coupled and put in two more interstages. I removed the plate resistors and cap coupling and it sound great. My question was about the grid leak resistors(as some people call them). Most of the information I found said what you did, the ac grid ground goes thru the interstage. I realize that the grid leak resistors affect the riaa some but was wondering if they do anything else in a phono amp???
Since I posted this I removed the resistors and it seems to working fine not a huge difference in sound but will recheck the riaa and see how much it changed and adjust that tommorrow.
Thanks Tom
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